Digital subscriber line (DSL) technologies are a family of high-speed network access technologies using ordinary telephone lines, with ADSL being an example of these technologies. ADSL is characterized by allowing a different downstream data rate from the service provider such as the telephone company to the consumer premises as compared to the upstream data rate from the customer premises to the service provider. The ADSL standard is described in S. Palm (ed.), “Asymmetrical Digital Subscriber Line (ADSL) Transceivers,” ITU-T Q4/SG15, G.992.1, 1999. This reference is incorporated herein by reference.
There are two primary benefits of DMT modulation on which ADSL is built. The first is the ability to do simplified equalization when the time span of the channel impulse response is shorter than the length of the cyclic prefix. The second is the ability to optimize the bit and gain allocation to maximize capacity.
Service providers are anxious to both increase their coverage area and offer higher data rates. Thus it is highly desirable to provide increased reach for long loops and increased rate for short loops. There is nothing in the ADSL standard which address these issues in a meaningful way.